Take the standard 2-D lattice packing of pennies; a(n) = number of ways to pick n pennies (modulo rotations and reflections) such that if we form a linkage with centers of pennies as hinges and with struts between centers of two touching pennies, the linkage is rigid.
1, 1, 1, 1, 1, 3,
1 seqfan posts
Mon Dec 14 05:02:26 CET 2009 [seqfan] a question about arrangements of pennies